1. Field of the Invention
The present invention generally relates to pressure swing adsorption systems. More specifically, the present invention relates to a monitoring and control system for pressure swing adsorption systems.
2. Discussion of the Background
Pressure swing adsorption (PSA) systems are used for the separation of one or more gases from a mixture of gases. A PSA apparatus usually includes multiple pressure vessels filled with a suitable adsorbent/adsorbents, with each vessel subsequently connected to two or more on-off valves that sequentially admit and expel gases at different pressures in order to affect the gas separation. The PSA cycle is defined by adjusting the time the vessel is open to the product channel relative to the time required to regenerate the adsorbent within each vessel. Given the cyclic nature of a PSA system, multiple vessels operating out of phase with each other is required to maintain constant product delivery. The regeneration stages often involve the exchange of gas between different vessels to perform pressure equalization and purge.
An important aspect of multiple vessel PSA systems is the implementation of pressure equalization to conserve pressure energy. The process of equalizing the pressures between two vessels, rather than expelling the pressure to the waste gas channel, improves the recovery of the lightly adsorbed component. Pressure equalization is performed when gas from a first pressure vessel, at a high pressure, is directed through an on-off valve into a section of pipe (referred to here as the “equalization channel”), fills the equalization channel and is then directed into a second vessel, at a lower pressure, through a second on-off valve. During this pressure equalization stage all remaining on-off valves that connect other vessels to the equalization channel are closed. The first vessel, which is providing gas, decreases in pressure, while the second vessel, which is receiving gas, increases in pressure until the two vessels reach a common final pressure.
The PSA system described in U.S. Pat. No. 6,699,307 to Lomax et al. discloses a seven vessel PSA system with three pressure equalizations. During the first pressure equalization stage, a vessel at high pressure, p1, is opened to the equalization channel at the same time a vessel at a lower intermediate pressure, p3, opens to the equalization channel. Gas flows through the equalization channel until the two vessels reach an intermediate pressure, p2, where p3<p2<p1. The second pressure equalization stage opens the vessel that decreased in pressure to p2 to the equalization channel at the same time another vessel at a lower intermediate pressure, p4, opens to the equalization channel. Gas flows through the equalization channel until the two vessels reach an intermediate pressure, p3, where p4<p3<p2. The third pressure equalization stage opens the vessel that decreased in pressure to p3 to the equalization channel at the same time another vessel at a lower intermediate pressure, p6, opens to the equalization channel. Gas flows through the equalization channel until the two vessels reach an intermediate pressure, p4, where p6<p4<p3. As pressure decreases in the vessel providing equalization gas, the capacity of the adsorbent to retain the impurities in the adsorbed phase decreases according to the equilibrium isotherm and the composition front of each impurity continues to move towards the discharge end where gas is being withdrawn. It is a desirable condition of pressure equalization to prevent the breakthrough of impurities into the equalization channel by providing enough adsorbent mass to take up the propagation of these composition fronts.
Purge is another important aspect of multiple vessel PSA systems to maintain purity of the product gas. If the purge is insufficient, then the impurities desorbed during the depressurization stage are not swept out of the void space, consequently polluting the product on the next production stage. However, if the purge is too great, then the volume of valuable product gas passed back through the vessel is in excess of that required to clean the void space of desorbed impurities at the desired purity level. Over purge results in an undesirable drop in product recovery.
In the PSA cycle described in U.S. Pat. No. 6,699,307 to Lomax et al., the vessel providing purge is progressively decreasing in pressure from some intermediate pressure, p4, to a final pressure, p5, where p4>p5. The purge stage requires the ratio of purge-to-feed gas to be sufficient to maintain product purity, which requires p5 to be manipulated in order to attain the required volume of gas passed out of the providing vessel. Although breakthrough of gas impurities during pressure equalization is undesirable, a small amount of breakthrough can happen during purge as pressure further decreases to achieve the desired purge-to-feed ratio. Therefore a time-varying composition that starts relatively clean and progressively increases in impurity levels is passed into a pipe connecting these vessels (referred to here as the “purge channel”). This gas, which has been passed into the purge channel, is expanded across a flow constriction device and then directed into the top of a second vessel that has undergone depressurization and is at the lowest pressure of the cycle. According to the equilibrium isotherm, the lowest pressure during the cycle will concentrate most of the impurities in the void space surrounding the adsorbent. The receive purge stage subsequently involves the sweeping of these impurities from the void space into the waste gas channel of the PSA system using gas directed from the provide purge vessel. In addition, the provide purge stream, enriched in the lightly adsorbed component, further assists desorption of the impurities by reducing their concentration surrounding the adsorbent in the void space.
The gas remaining in the purge channel at the end of the previous purge coupling is heavier in impurities than it was when the provide purge vessel first opened to the purge channel. Therefore, the section of the purge channel between the vessel providing and receiving purge must be flushed by pushing this initially impure gas into the receiving vessel until the flow of gas discharged from the provide purge vessel reaches the receiving vessel. The inventors have determined that if the distance between vessels providing and receiving purge is not held uniform, then the time-varying composition received into the receive purge vessel will also vary. The inventors have determined that his will result in one or several vessels in the system being out of balance with the others in terms of final axial composition through the vessel at the end of purge, potentially resulting in off-specification gas being sent to the product channel on the next production stage.
Comparing multi-vessel PSA systems that invoke, provide, and receive purge in the above manner shows several problems arise with this offset distance between vessels exchanging gas through the purge channel. For example, U.S. Pat. No. 3,986,849 to Fuderer et al. describes a ten vessel system that couples vessels providing and receiving purge at a distance of two vessels apart, with the exception of two of the vessels therein in which the purge channel distance jumps to eight vessels. U.S. Pat. No. 4,315,759 to Benkmann describes a nine vessel system where the distance between coupled purge vessels is two vessels apart, with the exception of two stages where this distance increases to seven vessels. U.S. Pat. No. 6,565,628 to Xu et al. describes a sixteen vessel system where the distance between coupled purge vessels is four vessels apart, with the exception of four stages where this distance extends to twelve vessels. The inventors have determined that such systems will result in one or more vessels being out of balance with the others in terms of final axial composition through the vessel at the end of purge, potentially resulting in off-specification gas being sent to the product channel on the next production stage.